It is known in the state of the art that currently there are many asynchronous short-circuited rotor generators, such as the so-called squirrel cage rotor, coupled to turbines, such as wind turbines, and connected directly to a three-phase electric power distribution grid by voltage step-up transformers. Consequently, said configuration of turbine connected to a generator is used to produce electric power that reaches end users through the three-phase electric power distribution grid.
Asynchronous short-circuited rotor generators, i.e., squirrel cage, are widely used because they are simple, robust, and relatively inexpensive. However, such squirrel cage generators also have disadvantages, such as high current demand during startup requiring a soft start function, a minimal ability to vary the rotational speed of the turbine because of a stiff characteristic torque versus rotational speed in the stable operation region, with resulting significant oscillations of the electromagnetic torque and of the active power transmitted to the electrical system, the inability to meet a requirement for dynamic reactive power exchange from the distribution grid for proper operation, the inability of starting up and operating as a stand-alone system, the inability to be insulated from the external power oscillations from the distribution grid, and the inability to damp such power oscillations.
It would be highly desirable to have a squirrel-cage generator which retained the features of simplicity, robustness. and relative low cost without the disadvantages discussed above.